A classical amplitude modulation (AM) radio receiver received a modulated radio frequency (RF) signal through an antenna, tuned a frequency of interest in the radio frequency down to baseband, and demodulated the signal. For example, U.S. entertainment AM radio includes frequencies in the range of 520 kilohertz (kHz) to 1680 kHz. The classical AM receiver used an antenna capable of receiving frequencies over this frequency band. This receiver mixed the signal down to baseband using a sinusoidal wave generated by a tuning circuit. The receiver then demodulated the baseband signal by envelope detection.
A listener tuned the classical receiver by turning a large knob, which was connected to a variable capacitor or a variable resistor. The variable capacitance or resistance changed the resonance frequency of an oscillator circuit, which then changed the frequency of the signal modulated down to baseband. While the classical AM receiver has been known and used for several decades, it has a few problems. First, the tuning is inexact, and the user had to rely the clarity of the signal he heard from the loudspeaker rather than the markings on the tuning knob to precisely tune a desired frequency. Also, the resonance frequency tended to drift over time as the components heated up. In addition, these components were bulky and expensive.
Digital tuning offered a significant improvement over the analog tuning used in the classical receiver. In digital tuning, resistances or capacitances are discretely switched into the resonator circuit. This type of tuning allows a digital display to indicate the frequency tuned by the user, and thus is superior to the analog tuning knob. However, this type of digital tuning also requires a large number of components and thus requires a large amount of circuit board area, and it would be desirable to further reduce the cost and board space of a receiver using this digital tuning circuit.
Another advance in technology which improves upon the classical radio receiver is the emergence of digital technology. Digital technology allows the received signal to be converted to a digital signal in an analog-to-digital converter, and then demodulated in the digital domain. Converting the signal to digital form also allows noise filtering using digital signal processing techniques. However, a problem with this approach is that the tuning signal remains unsynchronized to the clock used by the analog-to-digital converter, resulting in phase error in the demodulated signals and ultimately reduced resolution of the signal. What is needed, then, is a new radio receiver which further reduces cost and improves performance over known receivers.